I just laid DITRA down in my bathroom using a modified thinset on a plywood subfloor. I had good coverage, and while the thinset probably could have been a bit thicker it held a notch very well. I'm assuming it needs time to set up, so how long should I wait before laying the tile?

I've laid tile over Ditra within a few hours without issue.

Immediately. Just watch that you don't have in on so thick that your knees causes depressions under the membrane. It's actually not the low spots, but the high spots they create next to them.

I then go against their recommendation and use modified ontop the Ditra. Many large format tiles and natural stone call for quality modified thinset or medium set mortar.
I've laid lots of tile over lots of ditra with buckets and buckets of Modified mortars. Never had any kind of issue with it.
A good modified is far superior to unmodified for most application, and I believe over ditra as well.

Actually ditra started putting out their own brand modified to address the issue with LFT.

Well that's sooner than I thought. Thanks, guys!

I tried pouring somma that stuff a couple months ago when it was too hot....couldn't get the stuff down before it got hard no matter now thin I mixed it.

You must be talking about something totally different Gary.
Ditra is a plastic uncoupling membrane that comes on a roll. You trowel out thinset and embed the ditra in the mud. Then tile directly on the ditra.

I have laid tile for 20 yrs, and I prefer cement board to ditra, lol. As mentioned above, whatever method u use, go with the better grade of thinset. I mainly use Bostic PM, or Bonsal thinset with flex additive. Latex Modified minimum. To address your question, the times ive used Ditra, I notch troweled thinset, rolled on the Ditra, and embedded it good into the mortar with a grout float, then waited until the next day to lay tile on it. Have never had any problems with it, but I mailly only use it on jobs where theres floor heat going in under tile. Good luck with your project